Skyrocketing crude oil prices coupled with a dramatic rise in worldwide energy demands and increased concerns regarding environmental pollution and global warming make clean renewable energy a necessity for future energy needs. Among all renewable energies, solar power is clean, abundant and distributed virtually everywhere around the world. Photovoltaic devices take advantage of the enormous solar energy source and directly transfer sunlight into electricity.
However, due to high production costs and low light-to-energy conversion efficiencies associated with conventional photovoltaic technology, photovoltaic devices have not yet gained widespread use. For example, traditional photovoltaic devices rely on costly high quality silicon (Si), an indirect bandgap material, and involve multi-step vacuum processes during production.
More recent attempts to reduce the costs associated with photovoltaic device production have involved implementing thin-film technology. Thin-film technology uses direct bandgap materials, such as amorphous Si, cadmium telluride (CdTe) and copper indium gallium selenide (CuInGaSe2 also commonly abbreviated as “CIGS”), as the light absorber. Direct bandgap absorbers have strong light absorption at a thickness of only a few micrometers (μm). Reduced thickness means reduced material and production costs.
Of the three above-mentioned thin-film materials, CIGS is particularly attractive. Namely, CIGS-based photovoltaic devices with an efficiency of as high as 19.5 percent (%) have been demonstrated (as compared with 16.5% and 12% efficiencies for CdTe and amorphous Si absorbers, respectively). In addition, in CIGS there is no toxic cadmium (Cd) involved as with CdTe, and there are no degradation issues as with amorphous Si.
Very-high-efficiency CIGS absorber layers have been achieved using vacuum-based deposition processes, such as the “three-stage process” adopted by the National Renewable Energy Lab (NREL) which is a vacuum co-evaporation process wherein individual metal sources of copper (Cu), indium (In), gallium (Ga) and selenium (Se) are evaporated toward a heated substrate. The carefully controlled metal fluxes deliver a desired amount of metals, which react at the substrate under an overpressure of Se and form the CIGS compound.
Vacuum-based deposition processes for CIGS compounds are, however, expensive. Namely, expensive vacuum equipment, a sophisticated flux control setup and constant heating during the deposition all contribute to high production costs. Further, material utilization is not very efficient as a significant portion of the evaporated material can become deposited on the vacuum wall instead of on the substrate. Material utilization is an important consideration when expensive metals, such as In, are involved.
In an attempt to reduce production costs, manufacturers have turned to low-cost solution-based deposition processes. Although successfully implemented, solution-based processes still face many challenges, such as low efficiency and complicated processing. For example, one of the highest efficiencies obtained for a CIGS absorber device using a solution-based deposition process is 13.6% (by International Solar Electric Technology, Inc. (ISET)). See, for example, V. K. Kapur et al., “Non-Vacuum Processing of CuIn1-xGaxSe2 Solar Cells on Rigid and Flexible Substrates Using Nanoparticle Precursor Inks,” Thin Solid Films 431-432, pp. 53-57 (2003), the disclosure of which is incorporated by reference herein. In the ISET process, nanosized metal oxide particles are used to form a precursor ink which is printed onto substrates to create oxide thin films. The ISET solution process however requires complicated procedures to prepare the precursors and employs a toxic selenization procedure to convert the oxide thin films to CIGS.
Electrodeposition is another solution-based approach to deposit either CIGS or metal precursors on a conducting substrate (see, for example, D. Guimard et al., “Copper Indium Diselenide Solar Cells Prepared by Electrodeposition,” Proceedings of the 29th IEEE Photovoltaic Specialists Conference, New Orleans, pp. 692-695 (May 21-24, 2002); M. E. Calixto et al., “Single Bath Electrodeposition of CuInSe2 and Cu(In,Ga)Se2 for Thin Film Photovoltaic Cells,” Proceedings of the 31st IEEE Photovoltaic Specialists Conference, Lake Buena Vista, Fla., pp. 378-381 (Jan. 3-7, 2005); and A. Kampmann et al., “Electrodeposition of CIGS on Metal Substrates,” Mat. Res. Soc. Symp. Proc., vol. 763, B8.5.1-B8.5.6 (2003), the disclosure of each of which is incorporated by reference herein). However, device efficiency is generally below 10% unless a post-deposition vacuum evaporation process is employed to correct the film compositional makeup (see R. N. Bhattacharya et al., “15.4% CuIn1-xGaxSe2-Based Photovoltaic Cells From Solution-Based Precursor Films,” Thin Solid Films, vol. 361-362, 396-399 (2000), the disclosure of which is incorporated by reference herein).
Some other solution-based approaches include spray processes, doctor blading, ink jet printing and spin-coating. Spray processes, in particular, offer high throughput and high material utilization, and can be used to produce large-area uniform thin films with good adhesion to the substrate. Deposition of chalcopyrite materials copper indium disulfide (CuInS2) and copper indium diselenide (CuInSe2 which is also commonly abbreviated as “CIS”) has been demonstrated using this method.
Most sprayed chalcopyrite photovoltaic devices utilize a “superstrate” structure as compared to a substrate structure. FIGS. 1A-B are diagrams comparing substrate and superstrate photovoltaic devices. Specifically, FIG. 1A is a diagram illustrating exemplary substrate photovoltaic device 100. Substrate photovoltaic device 100 includes molybdenum (Mo)-coated substrate 102, CIGS absorber layer 104 adjacent to substrate 102, buffer layer 106 adjacent to a side of CIGS absorber layer 104 opposite substrate 102 and transparent conductive contact 108 adjacent to a side of buffer layer 106 opposite CIGS absorber layer 104.
The substrate is typically a glass (e.g., soda-lime glass) metal foil or plastic substrate. Buffer layer 106 typically comprises cadmium sulfide (CdS). Transparent conductive contact 108 typically comprises intrinsic zinc oxide (ZnO) and a layer of transparent conductive oxide (TCO), such as aluminum-doped ZnO and indium tin oxide (ITO). During operation, light is incident toward photovoltaic device 100 from the transparent conductive contact side.
In contrast, FIG. 1B is a diagram illustrating exemplary superstrate photovoltaic device 150. Superstrate photovoltaic device 150 includes ZnO, ITO or tin oxide (SnO2)-coated substrate 152, buffer layer 154 adjacent to substrate 152, CIGS absorber layer 156 adjacent to a side of buffer layer 154 opposite substrate 152 and metal back contact 158 adjacent to a side of CIGS absorber layer 156 opposite buffer layer 154. The substrate is typically a glass substrate. Buffer layer 154 typically comprises CdS. During operation, light enters device 150 from the ZnO, ITO or SnO2-coated substrate side.
For CIGS-based photovoltaic devices, the substrate structure (FIG. 1A) has certain advantages over the superstrate structure (FIG. 1B) including the ability to achieve better grain growth on Mo-coated substrates. When the substrate is Mo-coated soda-lime glass, better grain growth in the CIGS absorber enhances diffusion of sodium (Na) from the soda-lime glass to the absorber layer which further improves device performance. Further, because favorable CIGS grain growth is generally achieved at high temperatures, a substrate structure allows high temperature heating of the absorber layer before depositing the buffer layer (and other layers). In contrast, the superstrate structure requires first depositing the buffer layer/growing the absorber layer then performing a high temperature anneal. Using high temperatures on the buffer and absorber layers may destroy the p-n junction therebetween. For these reasons, the substrate structure typically produces higher efficiency photovoltaic devices than the superstrate structure.
Three types of solutions have been implemented with conventional spray processes. The first type is simply an aqueous solution composed of copper dichloride (CuCl2), indium trichloride (InCl3) and thiourea or selenourea. See, for example, M. Krunks et al., “Growth and Recrystallization of CuInS2 Films in Spray Pyrolytic Process,” Applied Surface Science, vol. 142, pp. 356-361 (1999), the disclosure of which is incorporated by reference herein. The efficiency of the completed device is however generally below five % possibly due to the junction heating during processing (as a superstrate structure is commonly employed) and/or the introduction of undesired impurities of chlorine (Cl), carbon (C) and, when sprayed in ambient atmosphere, oxygen (O) as well. Further, indium oxide (In2O3) can be formed during the spray process which can disadvantageously create a shunting path for the device.
The second type of spray solution contains single metal-organic precursors such as (Ph3P)2Cu(SEt)2In(SEt)2 and [P(n-Bu)3]2Cu(SEt)2In(Set)2. Photovoltaic devices fabricated using this type of precursor have efficiencies of only 0.68% (see J. D. Harris et al., “Characterization of CuInS2 Films Prepared by Atmospheric Pressure Spray Chemical Vapor Deposition” Materials Science and Engineering, vol. B98, pp. 150-155 (2003), the disclosure of which is incorporated by reference herein). The third type of spray pyrolysis solution consists of metal or metal oxide (Cu and In) nanoparticles suspended in a precursor solution. A spray process is implemented to deposit the precursor solution on a substrate. A high temperature Se-containing environment is used to convert the metal oxides to chalcopyrite (see M. Kaelin et al., “Electrosprayed and Selenized Cu/In Metal Particle Films,” Thin Solid Films, vol. 457, pp. 391-396 (2004), the disclosure of which is incorporated by reference herein). A stable oxide impurity phase containing In2O3 is reported to exist which can provide a shunting path affecting device performance.
Therefore, improved, low-cost spray techniques for photovoltaic device production that offer large scale production capabilities while at the same time providing improved device performance and efficiency would be desirable.